The present invention relates to a brushless motor used in a fan of an on-vehicle air conditioner.
A typical fan used in an on-vehicle air conditioner includes an outer-rotor type brushless motor. The motor has a stator fixed to a case and a rotor located about the stator. When fed with exciting current, the stator rotates the rotor. The stator has a centerpiece, a core and a wire wound about the core.
The core wire of the stator receives exciting current from an exciting circuit mounted on a control substrate. The current generates a magnetic field, which, in turn, rotates the rotor. The control substrate is either integrated with the case and located in the vicinity of the case or is separated from the stator and the brushless motor.
The control substrate includes a choke coil, which is an element in the exciting circuit. The choke coil eliminates high frequency noise in exciting current supplied from the exciting circuit to the core wire. The choke coil is connected to a power source and an element of the exciting circuit in series.
Recently, there has been an increasing demand for smaller brushless motors with greater power. To increase the power of a brushless motor, it is necessary to increase the magnitude of exciting current supplied from the exciting circuit to the core wire.
Increasing the magnitude of the exciting current raises the temperature of the core wire and elements on the substrate. In other words, the core wire and the substrate are two heat sources in the motor. Therefore, if the stator and the control substrate are mounted on the motor case, that is, if the stator and the control substrate are close to each other, increasing the magnitude of the exciting current rapidly raises the temperature of the motor. The heat of the motor increases the electrical resistance of the core wire. This, in turn, decreases the current in the core wire and lowers the power of the motor. In some cases, the heat burns and breaks the core wire.
On the other hand, if the control substrate is located away from the stator, that is, if the two heat sources are far from each other, increasing the exciting current does not significantly increase the temperature of the motor. However, this construction requires relatively long wires to connect the stator and the substrate. Further, the current-carrying capacity of elongated connecting wires may limit the magnitude of the exciting current supplied to the core wire. This prevents the power of the motor from being increased to a desired level. The construction also requires a separate case for the control substrate, which enlarges the size of the brushless motor.
Increasing the exciting current for enhancing the power of the motor raises the temperature of the choke coil. The raised temperature increases the resistance of the choke coil and thus lowers the power of the motor. The heat also may burn and break the choke coil.
Enlarging the choke coil increases the current-carrying capacity of the choke coil thereby suppressing the temperature increase of the choke coil. However, enlarging the choke coil enlarges the control substrate. A larger substrate requires a larger case thereby increasing the size of the motor.
If the choke coil is omitted from the substrate, electromagnetic noise generated by exciting current generates radio noise in the car radio or causes other electronic interference.